Fix flash_erase bank selection logic

Which bank a page lies in depends whether there are 2 banks. If there are 2 banks, the flash page size is cut in half and there are now double the pages. Half the pages in bank1, half in bank2. Therefore, write the BKER bit based on the half_npages.

arch/arm/src/stm32/stm32g4xxx_flash.c

@@ -187,19 +187,16 @@ static inline void flash_erase(size_t page)
 
   modifyreg32(STM32_FLASH_CR, 0, FLASH_CR_PAGE_ERASE);
 
-  /* DBANK=0, Page range 0 to 128
-   * DBANK=1, Page range 0 to 255
-   */
-
   modifyreg32(STM32_FLASH_CR, FLASH_CR_PNB_MASK,
               FLASH_CR_PNB(page));
 
 #if (defined(CONFIG_STM32_STM32G47XX) || \
      defined(CONFIG_STM32_STM32G48XX))
+  uint32_t half_npages = get_flash_npages() / 2
 
   if (getreg32(STM32_FLASH_OPTR) & FLASH_OPTR_DBANK)
     {
-      if (page <= 0x7f)
+      if (page < half_npages)
         {
           /* Select bank 1 */